Internet Engineering Task Force (IETF) J. Levine
Request for Comments: 6783 Taughannock Networks
Obsoletes: 5983 R. Gellens
Category: Informational Qualcomm Incorporated
ISSN: 2070-1721 November 2012 Mailing Lists and Non-ASCII Addresses
This document describes considerations for mailing lists with the
introduction of non-ASCII UTF-8 email addresses. It outlines some
possible scenarios for handling lists with mixtures of non-ASCII and
traditional addresses but does not specify protocol changes or offer
implementation or deployment advice.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Mailing List Header Additions and Modifications . . . . . . 31.2. Non-ASCII Email Addresses . . . . . . . . . . . . . . . . . 32. Scenarios Involving Mailing Lists . . . . . . . . . . . . . . . 42.1. Fully SMTPUTF8 Lists . . . . . . . . . . . . . . . . . . . 42.2. Mixed SMTPUTF8 and ASCII Lists . . . . . . . . . . . . . . 52.3. SMTP Issues . . . . . . . . . . . . . . . . . . . . . . . . 53. List Headers . . . . . . . . . . . . . . . . . . . . . . . . . 63.1. SMTPUTF8 List Headers . . . . . . . . . . . . . . . . . . . 63.2. Downgrading List Headers . . . . . . . . . . . . . . . . . 73.3. Subscribers' Addresses in Downgraded Headers . . . . . . . 84. Security Considerations . . . . . . . . . . . . . . . . . . . . 85. References . . . . . . . . . . . . . . . . . . . . . . . . . . 85.1. Normative References . . . . . . . . . . . . . . . . . . . 85.2. Informative References . . . . . . . . . . . . . . . . . . 91. Introduction
This document describes considerations for mailing lists with the
introduction of non-ASCII UTF-8 email addresses. The usage of such
addresses is described in [RFC6530].
Mailing lists are an important part of email usage and collaborative
communications. The introduction of internationalized email
addresses affects mailing lists in three main areas: (1) transport
(receiving and sending messages); (2) message headers of received and
retransmitted messages; and (3) mailing list operational policies.
A mailing list is a mechanism that distributes a message to multiple
recipients when the originator sends it to a single address. An
agent, usually software rather than a person, at that single address
receives the message and then causes the message to be redistributed
to a list of recipients. This agent usually sets the envelope return
address (henceforth called the "bounce address") of the redistributed
message to a different address from that of the original message.
Using a different bounce address directs error and other
automatically generated messages to an error-handling address
associated with the mailing list. This sends error and other
automatic messages to the list agent, which can often do something
useful with them, rather than to the original sender, who typically
doesn't control the list and hence can't do anything about them.
In most cases, the mailing list agent redistributes a received
message to its subscribers as a new message, that is, conceptually it
uses message submission [RFC6409] (as did the sender of the original
message). The exception, where the mailing list is not managed by a
separate agent that receives and redistributes messages in separate
transactions but is implemented by an expansion step within an SMTP
transaction where one local address expands to multiple local or non-
local addresses, is not addressed by this document.
1.1. Mailing List Header Additions and Modifications
Some list agents alter message header fields, while others do not. A
number of standardized list-related header fields have been defined,
and many lists add one or more of these headers. Separate from these
standardized list-specific header fields, and despite a history of
interoperability problems from doing so, some lists alter or add
header fields in an attempt to control where replies are sent. Such
lists typically add or replace the "Reply-To" field, and some add or
replace the "Sender" field. Some lists alter or replace other
fields, including "From".
Among these list-specific header fields are those specified in RFCs
2369 [RFC2369] and 2919 [RFC2919]. For more information, see
1.2. Non-ASCII Email Addresses
While the mail transport protocol is the same for regular email
recipients and mailing list recipients, list agents have special
considerations with non-ASCII email addresses because they retransmit
messages composed by other agents to potentially many recipients.
There are considerations for non-ASCII email addresses in the
envelope as well as in header fields of redistributed messages. In
particular, a message with non-ASCII addresses in the headers or
envelope cannot be sent to non-SMTPUTF8 recipients.
With mailing lists, there are two different types of considerations:
first, the purely technical ones involving message handling, error
cases, and the like, and second, those that arise from the fact that
humans use mailing lists to communicate. As an example of the first,
list agents might choose to reject all messages from non-ASCII
addresses if they are unprepared to handle SMTPUTF8 mail. As an
example of the second, a user who sends a message to a list often is
unaware of the list membership. In particular, the user often
doesn't know if the members are SMTPUTF8 mail users or not, and often
neither the original sender nor the recipients personally know each
other. As a consequence of this, remedies that may be readily
available for one-to-one communication might not be appropriate when
dealing with mailing lists. For example, if a user sends a message
that is undeliverable, normally the telephone, instant messaging, or
other forms of communication are available to obtain a working
address. With mailing lists, the users may not have any recourse.
Of course, with mailing lists, the original sender usually does not
know which list members successfully received a message or if it was
undeliverable to some.
Conceptually, a mailing list's internationalization can be divided
into three capabilities. First, does the list have a non-ASCII
submission address? Second, does the list agent accept subscriptions
for addresses containing non-ASCII characters? And third, does the
list agent accept messages that require SMTPUTF8 capabilities?
If a list has subscribers with ASCII addresses, those subscribers
might or might not be able to accept SMTPUTF8 messages.
2. Scenarios Involving Mailing Lists
Generally (and exclusively within the scope of this document), an
original message is sent to a mailing list as a completely separate
and independent transaction from the list agent sending the
retransmitted message to one or more list recipients. In both cases,
the message might be addressed only to the list address or might have
recipients in addition to the list. Furthermore, the list agent
might choose to send the retransmitted message to each list recipient
in a separate message submission transaction or might choose to
include multiple recipients per transaction. Often, list agents are
constructed to work in cooperation with, rather than include the
functionality of, a message submission server; hence, the list
transmits to a single submission server one copy of the retransmitted
message. The submission server then decides which recipients to
include in which transaction.
2.1. Fully SMTPUTF8 Lists
Some lists may wish to be fully SMTPUTF8. That is, all subscribers
are expected to be able to receive SMTPUTF8 mail. For list hygiene
reasons, such a list would probably want to prevent subscriptions
from addresses that are unable to receive SMTPUTF8 mail. If a
putative subscriber has a non-ASCII address, it must be able to
receive SMTPUTF8 mail, but there is no way to tell whether a
subscriber with an ASCII address can receive SMTPUTF8 mail short of
sending an SMTPUTF8 probe or confirmation message and somehow finding
out whether it was delivered, e.g., if the user clicked a link in the
2.2. Mixed SMTPUTF8 and ASCII Lists
Other lists may wish to handle a mixture of SMTPUTF8 and ASCII
subscribers, either as a transitional measure as subscribers upgrade
to SMTPUTF8-capable mail software or as an ongoing feature. While it
is not possible in general to downgrade SMTPUTF8 mail to ASCII mail,
list software might divide the recipients into two sets, SMTPUTF8 and
ASCII recipients, and create a downgraded version of SMTPUTF8 list
messages to send to ASCII recipients. See Sections 3.2 and 3.3.
To determine which set an address belongs in, list software might
make the conservative assumption that ASCII addresses get ASCII
messages, it might try to probe the address with an SMTPUTF8 test
message, or it might let the subscriber set the message format
manually, similar to the way that some lists now let subscribers
choose between plain text and HTML mail, or individual messages and a
To determine whether a message needs to be downgraded for ASCII
recipients, list software might assume that any message received via
an SMTPUTF8 SMTP session is an SMTPUTF8 message or might examine the
headers and body of the message to see whether it needs SMTPUTF8
treatment. Depending on the interface between the list software and
the Mail Transfer Agent (MTA) and Mail Delivery Agent (MDA) that
handle incoming messages, it may not be able to tell the type of
session for incoming messages.
2.3. SMTP Issues
Mailing list software usually changes the envelope addresses on each
message. The bounce address is set to an address that will return
bounces to the list agent, and the recipient addresses are set to the
subscribers of the list. For some lists, all messages to a list get
the same bounce address. For others, list software may create a
bounce address per recipient or a unique bounce address per message
per recipient, bounce management techniques known as Variable
Envelope Return Paths or VERP [VERP].
The bounce address for a list typically includes the name of the
list, so a list with a non-ASCII name will have a non-ASCII bounce
address. Given the unknown paths that bounce messages might take,
list software might instead use an ASCII bounce address to make it
more likely that bounces can be delivered back to the list agent.
Similarly, a VERP address for each subscriber typically embeds a
version of the subscriber's address so the VERP bounce address for a
non-ASCII subscriber address will be a non-ASCII address. For the
same reason, the list software might use ASCII bounce addresses that
encode the recipient's identity in some other way.
3. List Headers
List agents typically add list-specific headers to each message
before resending the message to list recipients.
3.1. SMTPUTF8 List Headers
The list headers in RFCs 2369 [RFC2369] and 2919 [RFC2919] were all
specified before SMTPUTF8 mail existed, and their definitions do not
address where non-ASCII characters might appear. These include, for
List-Id: List Header Mailing List
<mailto:firstname.lastname@example.org> (Contact Person for Help)
As described in [RFC2369], "[t]he contents of the list header fields
mostly consist of angle-bracket ('<', '>') enclosed URLs, with
internal whitespace being ignored". [RFC2919] specifies that "[t]he
list identifier will, in most cases, appear like a host name in a
domain of the list owner". Since these headers were defined in the
context of ASCII mail, these headers permit only ASCII text,
including in the URLs.
The most commonly used URI schemes in List-* headers tend to be http
and mailto [RFC6068], although they sometimes include https and ftp
and, in principle, can contain any valid URI.
Even if a scheme permits an internationalized form, it should use a
pure ASCII form of the URI described in [RFC3986]. Future work may
extend these header fields or define replacements to directly support
unencoded non-ASCII outside the ASCII repertoire in these and other
header fields, but in the absence of such extension or replacement,
non-ASCII characters can only be included by encoding them as ASCII.
The encoding technique specified in [RFC3986] is to use a pair of hex
digits preceded by a percent sign, but percent signs have been used
informally in mail addresses to do source routing. Although few mail
systems still permit source routing, a lot of mail software still
forbids or escapes characters formerly used for source routing, which
can lead to unfortunate interactions with percent-encoded URIs or any
URI that includes one of those characters. If a program interpreting
a mailto: URI knew that the Mail User Agent (MUA) in use were able to
handle non-ASCII data, the program could pass the URI in unencoded
non-ASCII, avoiding problems with misinterpreted percent signs, but
at this point, there is no standard or even informal way for MUAs to
signal SMTPUTF8 capabilities. Also, note that whether
internationalized domain names should be percent-encoded or appear in
A-label form [RFC5890] depends on the context in which they occur.
The List-ID header field uniquely identifies a list. The intent is
that the value of this header remain constant, even if the machine or
system used to operate or host the list changes. This header field
is often used in various filters and tests, such as client-side
filters, Sieve filters [RFC5228], and so forth. If the definition of
a List-ID header field were to be extended to allow non-ASCII text,
filters and tests might not properly compare encoded and unencoded
versions of a non-ASCII value. In addition to these comparison
considerations, it is generally desirable that this header field
contain something meaningful that users can type in. However, ASCII
encodings of non-ASCII characters are unlikely to be meaningful to
users or easy for them to accurately type.
3.2. Downgrading List Headers
If list software prepares a downgraded version of an SMTPUTF8
message, all the List-* headers must be downgraded. In particular,
if a List-* header contains a non-ASCII mailto (even encoded in
ASCII), it may be advisable to edit the header to remove the non-
ASCII address or replace it with an equivalent ASCII address if one
is known to the list software. Otherwise, a client might run into
trouble if the decoded mailto results in a non-ASCII address. If a
header that contains a mailto URL is downgraded by percent encoding,
some mail software may misinterpret the percent signs as attempted
When downgrading list headers, it may not be possible to produce a
downgraded version that is satisfactorily equivalent to the original
header. In particular, if a non-ASCII List-ID is downgraded to an
ASCII version, software and humans at recipient systems will
typically not be able to tell that both refer to the same list.
If lists permit mail with multiple MIME parts, some MIME headers in
SMTPUTF8 messages may include non-ASCII characters in file names and
other descriptive text strings. Downgrading these strings may lose
the sense of the names, break references from other MIME parts (such
as HTML IMG references to embedded images), and otherwise damage the
3.3. Subscribers' Addresses in Downgraded Headers
List software typically leaves the original submitter's address in
the From: line, both so that recipients can tell who wrote the
message and so that they have a choice of responding to the list or
directly to the submitter. If a submitter has a non-ASCII address,
there is no way to downgrade the From: header and preserve the
address so that ASCII recipients can respond to it, since non-
SMTPUTF8 mail systems can't send mail to non-ASCII addresses.
Possible work-arounds (none implemented that we know of) might
include allowing subscribers with non-ASCII addresses to register an
alternate ASCII address with the list software, having the list
software itself create ASCII forwarding addresses, or just putting
the list's address in the From: line and losing the ability to
respond directly to the submitter.
4. Security Considerations
None beyond what mailing list agents do now.
5.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
URI Scheme", RFC 6068, October 2010.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
5.2. Informative References
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[VERP] Bernstein, D., "Variable Envelope Return Paths",
February 1997, <http://cr.yp.to/proto/verp.txt>.
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